Oxidation inhibitor for gas odorants



Patented Sept. 24, 1957 Elmer E. Johnson,

Richmond, Calif., assignors to California Research Corporation, San Francisco, Calif., a corporation of Delaware No Drawing. Application December 27, 1952, Serial No. 328,254

2 Claims. (Cl. 252408) The present invention relates to an improvement in the manufacture of odorants for gaseous fuels such as natural gas and other heating and illuminating gas.

\ Among the various odoriferous materials employed to impart a characteristic odor to combustible gases used in the industry-and in the home for heating and cooking purposes, thereby warning of the presence of gas leaks and of the consequent existence of toxic and explosive hazards, volatile liquid mercaptans boiling in the range from about 7 to about 130 C. have been recognized as very effective warning agents, since their presence in the air is readily noticeable by most people in concentrations as low as 0.25 part per million.

Although these liquid mercaptans containing from one to five carbon atoms per molecule are efiective warning agents, their employment in combustible gases involves certain difliculties:

First of all, rather rigid specifications are set up in the industry with respect to the purity of the mercaptans suitable for odorizing gaseous fuels. Usually, a minimum purity of 95 to 96% by volume is specified, and in the case of ethyl mercaptan and some other mixtures of mercaptans a purity of 98% and even higher is often specified. Liquid mercaptans are, however, susceptible to oxidation which is responsible for the conversion of a portion of the mercaptan product into disulfides lacking the desired odor intensity of the mercaptans and, therefore, unsuitable as odorants. These disulfides, together with the small amounts, from 1 to of inert hydrocarbons ordinarily remaining in the mercaptan mixtures after their separation and purification, constitute the so-called oil content of the mercaptan, expressed in per cent by volume thereof.

Mercaptans whether pure or as mixtures are particularly susceptible to oxidation during the transfer from the manufacturing plant into barrels, storage tanks, or tank cars as well as during use as odorants. While the mercaptans remain stored for long periods of time, during which the container may be opened several times to remove portions of its contents, additional air oxygen comes into contact with the mercaptans and oxidizes them still further, thereby increasing their oil content. In view of the rigid specifications, mercaptan odorants which contain more than 1 to 5% oil (hydrocarbon plus disulfide) as a result of such an oxidation do not pass the specifications and are not accepted by the purchaser.

Another important drawback attending the application of a liquid mercaptan in the odorization of combustible gases (natural and synthetic fuel gases) is the fact of corrosion of the iron and copper equipment such as storage tanks, barrels, pipe lines, etc., owing to the acid nature of the odorant, i. e., the invariable presence of traces of hydrogen sulfide. Furthermore, because of the sul- El Cerrito, and Henry A. Francis,

fur content of these mercaptan materials, corrosive decomposition products are formed in the hot areas of the iron and copper appliances burning fuel gases containing these odorants and cause deterioration of the metal parts. In fact, copper and high copper alloys are corroded by mercaptans even at room temperature. This rather extensive corrosion is furthermore enhanced by the catalytic action of iron in contact with liquid mercaptans.

It might appear that control of these tendencies of mercaptans to oxidize and to corrode iron and copper could be readily accomplished by adding an oxidation inhibitor to suppress disulfide formation and a corrosion inhibitor to suppress the attack on materials. However, exploration of this route to control the undesirable properties of mercaptans develops a peculiar situation. The oxidation inhibitors generally increase the rate of corrosion, and the corrosion inhibitors generally increase the rate of disulfide formation. We found that a single pair of these materials, namely, guaiacol, an oxidation inhibitor, and reduced nitrogen bases, a corrosion inhibitor, can be used together to suppress both corrosion and disulfide formation, although individually they promote corrosion and disulfide formation, respectively. Only a small quantity of guaiacol and reduced nitrogen bases, usually in a weight ratio of 1.5 to 2.5 parts of the guaiacol to 1 part of nitrogen bases, is required to secure the desired inhibition of oxidation and reduction of corrosiveness. Ordinarily, an inhibitor composition containing from 0.001 to 0.1% by weight of guaiacol and 0.001 to 0.1% by weight of reduced nitrogen bases, based on the weight of the mercaptan odorant, is sufficient to reduce drastically the oxidation tendency and corrosiveness of the mercaptans. Larger amounts can be added to the mercaptans, but the incremental addition produces little additional inhibitory effect. This finding is entirely unexpected. Guaiacol, when added alone in comparable amounts to mixtures of liquid mercaptans, increases the corrosion of iron. On the other hand, reduced nitrogen bases, when employed alone in comparable amounts in mercaptan mixtures, increase the inert oil content thereof, even though reducing the corrosion of iron and copper. Consequently, the substantial reduction of both the oxidation tendency and the corrosiveness of the mercaptans with respect to iron and copper is surprising.

The reduced nitrogen bases suitable for the formulation of the inhibitor composition of the present invention are secondary amines produced by hydrogenation of petroleum nitrogen bases recoverable from straight-run or cracked petroleum distillates, in accordance with the technique described in U. S. Patent No. 2,302,655, to J. T. Rutherford, while guaiacol is the commercial high purity guaiacol. The introduction of these ingredients into the mercaptan mixtures offers no difficulty and is effected by conventional procedures.

An extensive series of tests were carried out as follows: A 60 gram sample of mercaptan was placed into a 4 ounce glass bottle, and strips of iron and copper were immersed into the liquid material. The bottle was closed by means of a vented cork and left to stand for 7 days. Each day air was periodically blown through the mercaptan solution for a short period of about 15 minutes duration by means of an aspirator bulb. These tests have clearly demonstrated the superior effect of the: guaiacolreduced nitrogen bases combination in simultaneously inhibiting the oxidation of mercaptans and corrosion of iron and copper.

Several representative" examples of t hese series are as-. sembled in Table I to illustrate-the improvement 'obtained with the aid of the combination'of guaiacol and reduced nitrogen bases.

said guaiacol to said secondary amines ranging from 2. A mercaptan composition, non-corrosive to iron and copper and resistant to oxidation, consisting essentially From thetabulated data it 'is seenthat the combinanonetreduced nitrogen bases and'guaiaool reducescorrosion 'and'disulfide formation more than either one of the ingredients a1-one, as measured by the corrosion'rate and netoil increase values.

In conclusion, it-is to be' understood that'the' inhibitor combination of reduced nitrogen bases and guaiacoldescribed 'hereiuis not 'limitedjin its application'to mer captan'odorants for use in gaseous fuels, but may also'be employed in many other situations involving a liquid mercaptan susceptible tooxi-dation and capable of causing corrosion of iron and copper receptacles, for instance, during the storage of methyl mercaptan prior to its utili- Zation in the production of methionine.

We claim:

1.-A stable non-corrosive odorant consisting essentially of a mercaptan containing'from 1 L05 carbon'atoms'per molecule, from 0.001 to 0.1% 'by weight, based on the mercaptans, of guaiacol, and from 0.001 to 0.1% by weight, based on the mercaptan, of secondary amines obtained by reducing'petroleum nitrogen bases; the ratio of of a liquid mercaptan containingfromjlto-5 carbon atoms permolecule and about 0.003 by weight, based on the mercaptan, of a mixture of :guaiacol and secondary amines obtained by reducing petroleum nitrogen bases, the'weight ratio of said guaiacolto saidsecondary amines? in the mixture ranging from 1.5:1' to 2.5:-l. f

References Cited in the" fileof this-patent UNITED" STATES PATENTS 1,986,333 'Fulw'eiler Jan. 1, 1 935;

2,032,431 Odell Mar. 3, 193 7 2,068,614 Thomas et al. l. Jan."19, 19 37 2,302,655 Rutherford et a1. Nov. 17,1942 2,313,531 Figg et 'al. L; Mar. 9,1943; 2,430,050 Gill Nov; 4,1947

OTHER REFERENCES V Warning Agents for Methyl Chloride in Refrigerationi System," publicationof The Roessl'er and Hasslacher; Chemical 00.,1930, pp. 30 and 31. l 

1. A STABLE NON-CORROSIVE ODORANT CONSISTING ESSENTIALLY OF A MERCAPTAN CONTAINING FROM 1 TO 5 CARBON ATOMS PER MOLECULE, FROM 0.001 TO 0.1% BY WEIGHT, BASED ON THE MERCAPTANS, OF GUAIACOL, AND FROM 0.001 TO 0.1% BY WEIGHT, BASED ON THE MERCAPTAN, OF SECONDARY AMINES OBTAINED BY REDUCING PETROLEUM NITROGEN BASES, THE RATIO OF SAID GUAIACOL TO SAID SECONDARY AMINES RANGING FROM 1.5:1 TO 2.5:1. 